Cord factor (trehalose 6-6' dimycolate). COAM (chlorite-oxidized oxyamylose), zymosan, glucan, Salmonella enteritidis 11RX and Listeria monocytogenes were found to protect mice against subsequent infection with Babesia microti, an intra-erythrocytic protozoan parasite. This protection was not observed after injection of Staphylococcus epidermidis, a viridans group Streptococcus, thioglycollate, or colloidal carbon. All the agents which protect against B. microti have also been reported to induce non-specific protection against experimental tumours. The parasites appear to die inside circulating red cells. This implies that these can exert non-specific protection against this parasite through the mediation of a soluble factor.